Negative emotionality, as well as attachment security and disorganization, are seen as major contributors to social adjustment and maladjustment in childhood. However, relatively little is known about whether infant negative emotionality and attachment quality operate together to affect developing behavior problems. The present study thus aims to contribute to this question. Participants were 64 healthy firstborn children and their primary caregivers. Negative emotionality was assessed at the infant ages of 4, 8, and 12 months using laboratory routines. At 18 months, the Strange Situation procedure was conducted to assess infant attachment security and disorganization, and at 30 months, the child's behavior problems were assessed within a structured clinical interview. Attachment security and attachment disorganization were significantly associated with subsequent behavior problems. There was no significant relation between infant negative emotionality and behavior problems. However, there were indications of a stronger association between attachment disorganization and behavior problems in infants high in negative emotionality. The results underpin the importance of attachment quality as well as negative emotionality in social adjustment. Disorganized attachment precedes poor adjustment, especially in infants high in negative emotionality.